It is well-known that an aqueous solution in contact with an untreated metal substrate can result in corrosion of the untreated metal substrate. Therefore, a metal article, such as a metal container or a metal closure for a glass or plastic container, like a food product, is rendered corrosion resistant in order to retard or eliminate interactions between the aqueous product and the metal article. Generally, corrosion resistance is imparted to the metal article, or to a metal substrate in general, by passivating the metal substrate, or by coating the metal substrate with a corrosion-inhibiting coating.
Investigators continually have sought improved coatings to reduce or eliminate the corrosion of metal substrate. For example, investigators have sought to improve the imperviousness of the coating in order to prevent corrosion-causing ions, oxygen molecules and water molecules from contacting and interacting with the metal substrate. Imperviousness can be improved by providing a thicker, more flexible and more adhesive coating, but often, improving one particular advantageous property is achieved at the expense of another advantageous property. For example, if the adhesive properties of a coating is improved, the flexibility of the coating can be adversely affected.
In addition, practical considerations limit the thickness, adhesive properties and flexibility of a coating applied to a metal substrate. For example, thick coatings are expensive, require a longer cure time, can be esthetically unpleasing and can adversely affect the process of stamping and molding the coated metal substrate into a useful metal article. Similarly, the coating should be sufficiently flexible such that the continuity of the coating is not destroyed during stamping and molding of the metal substrate into the desired shape of the metal article.
Corrosion-inhibiting compounds have been included in coating compositions to interact, either chemically or electrochemically, with the corrosion-causing agents or with the metal surface in order to retard the corrosion process. Traditionally, chromate compounds and lead compounds were used to retard and inhibit corrosion of metal substrates. However, both types of compounds introduce environmental and toxicological concerns making their use in coatings, and especially in coatings for metal articles that contact food, undesirable.
Inorganic extender pigments, such as calcium carbonate, talc, aluminum flake or mica, also have been included in coating compositions to inhibit the ability of water, oxygen and other corrosion-causing agents from contacting and interacting with the metal substrate. Investigators recently have used organic corrosion-inhibiting compounds to retard the corrosion of metal substrates Originally, the zinc and lead salts of a hydroxy- or mercapto-containing five or six membered heterocyclic compound, such as the zinc or lead salts of 2-mercaptobenzothiazole, were utilized. However, such corrosion-inhibiting compounds did not overcome the environmental and toxicological disadvantages of including a heavy metal in the composition.
Berner et al., in U.S. Pat. No. 4,612,049, disclose organic corrosion inhibiting compounds that can be used in a coating composition for metal substrates. Berner et al. generally teach that certain benzoxazoles, benzthiazoles and benzimidazoles can be combined with a resinous film-forming binder to provide a coating composition that inhibits corrosion of metal substrates. The Berner et al. patent teaches general corrosion-inhibiting compositions that include an organic corrosion inhibitor, but the Berner et al. patent does not teach or suggest particular corrosion-inhibiting coating compositions that further demonstrate, after curing, the properties of improved flexibility and excellent adhesion to both the metal substrate and to a variety of topcoats applied over the cured corrosion-inhibiting composition. As will be discussed more fully hereinafter, the improved adhesion between the cured corrosion-inhibiting composition and a variety of topcoats allows a more efficient processing of the coated metal substrate into a shaped metal article, like a metal container or a metal closure, wherein the shaped metal article effectively resists corrosion resulting from contact with aqueous liquids, and especially with acidic aqueous liquids that include volatile acids.
Braig U.S. Pat. No. 4,818,777 and Braig et al. U.S. Patent No. 4,894,091 also disclose organic corrosion-inhibiting compounds useful in coatings and related compositions. The publication, "A New, Organic Corrosion Inhibitor for Coatings", presented by R.A. Behrens and A. Braig, at the Water-Borne and Higher Solids Coatings Symposium, New Orleans, LA., Feb. 3.5, 1988, describes the corrosion process, and the coatings and corrosion-inhibiting compounds used to retard or eliminate the corrosion of metal substrates.
Although the above-identified patents and publication disclose effective organic corrosion inhibiting compounds, these references do not teach particular coating compositions that, after curing,: (1) effectively inhibit corrosion, (2) demonstrate improved adhesion both to a metal substrate and to a variety of types of polymeric topcoats applied over the cured corrosion-inhibiting composition, and (3) demonstrates substantially improved flexibility even after extended cure times of about one hour at 400.degree. F. As an added advantage, it has been found that a present corrosion-inhibiting coating composition, after application as a primer coat on a surface of a metal substrate and subsequent curing, effectively inhibits corrosion of the metal substrate, even if only a single topcoat is applied over the cured primer coat. Conventionally, because prior primer coats either did not exhibit a sufficient corrosion inhibiting ability or exhibit sufficient adhesion to a variety of topcoats and metal substrates, a primer coat was chosen, in part, for its ability to adhere to a particular topcoat, and often two topcoats were applied over the primer coat to achieve sufficient corrosion inhibition. Accordingly, because of improved corrosion-inhibiting properties and because of improved flexibility and adhesion to a variety of types of topcoats, a corrosion-inhibiting coating composition of the present invention has a more universal range of applications, such as for the interior coating of a metal container for holding food products and for the primer coat on the interior of a metal closure for a glass or plastic container for holding food products.